Planinsko polje is a polje (karsts feature) in the Notranjska region. It is one of the most typical poljes. River Unica flows over the field with the 8 km long stream. After the heavy rain the field is flooded and it becomes
a lake for some time period
. Water comes from the Cerknica direction (Rak River) and Postojna direction (Pivka River) from
(cave). Rivers Rak and Pivka join together 500 m inside Planinska jama and the water comes out from the Planinsk ajama as Unica river on the south part of Planinsko polje. Unica river flows underground again on the North-East part of the field. The river is a rich fish habitat, so the Unica River is known to the sport fishermen.